1. Field of the Invention
The present invention relates to a circuit for improving the short circuit withstand capability of an insulated gate bipolar transistor (IGBT) and preventing switching voltage transients from damaging the IGBT.
2. Description of the Related Art
Power transistors are used in electrical equipment to control and convert electrical power. The objective is achieved by switching these devices on and off, at predetermined instances. The devices are selected by system designers to reliably handle circuit currents under normal as well as estimated overload conditions. However, under fault or short circuit conditions, a device may be subjected to very high surge currents, the magnitude of which is limited mainly by its own gain. Only timely control and removal of fault current, by some external means, will save such device from failure.
In applications where system fault is very real, the motor-drive market being the prime example, external protection circuits are used to sense the fault and turn off the transistor by shutting down the base/gate drive. In all such applications, except where "intelligent" modules are used, the protection circuit is connected externally to the device.
Consequently, device manufacturers are expected to guarantee minimum short-circuit withstand time, which is a measure of how long a device will survive fault current. A device tradeoff exists between the short circuit withstand time and the current-gain of the power transistors. That is, the higher the gain of the transistor, the higher the fault current magnitude will be and the shorter the short circuit withstand time will be. The tradeoff, illustrated in FIG. 1, is more important for IGBT transistor manufacturers due to the inherently higher gain of these devices. The low-gain IGBTs available today allow longer short circuit time, but at the expense of operating efficiency. The high gain devices, on the other hand, boast greater efficiency, but require quicker external protection circuits.
The present market trend is to improve system efficiency. This, in turn, translates to demand for high efficiency IGBTs. The requirement for longer short circuit time (10 .mu.s) is still prevalent, especially for the existing designs. There is thus a pressing need for high efficiency (higher gain), long short circuit time IGBTs. The inherent device tradeoff, however, does not allow IGBTs to have it both ways.
Prior solutions for protecting IGBTs have been devised, but are not entirely satisfactory. For example, the prior art circuit shown in FIG. 2 uses a current sense IGBT to monitor the IGBT current. If the device current exceeds the preset value, the voltage developed across the sense-resistor R rises above the MOSFET's gate-threshold level. As the MOSFET starts to conduct, a voltage drop is developed across external series gate resistor R.sub.G. The IGBT gate voltage is accordingly reduced and the device current is regulated. The limitation of this circuit is that it can only be used with current-sense IGBTs. The cost effectiveness of manufacturing current-sense IGBTs and problems related to controlling tolerances in the current-sense ratio are major considerations. The vast majority of IGBTs used today in power modules are not of the current-sense type. Also, this circuit cannot be "plugged" externally to an IGBT module unless current-sense terminals are brought outside, a difficult task since this may render the circuit sensitive to system noise. Moreover, the circuit dependency on the value of R.sub.G compromises the IGBT turn-on losses.
The prior art circuit shown in FIG. 3 makes use of power ICs with a sense feature. The circuit monitors collector to emitter voltage to sense the fault. This circuit has built-in delays to allow for turn-on switching and narrow load current spikes. A fault condition lasting for longer than the above-described delay, however, is greeted with complete IGBT shutdown. The circuit has no provision to: 1) limit the initial high peak of fault current; or, 2) keep limiting the fault current, while restoring normal gate drive if the fault is of a short, transient type. The other disadvantage of this circuit is that it requires a DC voltage supply to operate. Thus, it cannot be simply inserted inside the existing modules.
Accordingly, a need exists for a circuit which improves the short circuit withstand capability of IGBTs without the above-noted disadvantages of the prior art.
Another problem with the recent development of high current, fast switching IGBT modules is that these modules may produce detrimental switching voltage transients. When a power transistor is suddenly turned off, trapped energy in the circuit produces stray inductance which is dissipated in the switching device, causing a voltage spike across the device. The magnitude of this transient voltage is proportional to the amount of stray inductance and the rate of the fall of the turn-off current. If a short circuit current is turned off too quickly, a destructive voltage transient is produced which could destroy the transistor.
Thus, there is also a need for a circuit which prevents switching voltage transients from destroying the high current IGBT during turn off of the IGBT under fault conditions.